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Self-Organised Criticality: Difference between revisions
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In magnetically confined plasmas, this state is thought to be responsible for the global transport phenomena of: | In magnetically confined plasmas, this state is thought to be responsible for the global transport phenomena of: | ||
* [[profile consistency]], which is the observation that profiles tend to have roughly the same shape, regardless of the power and location of the applied heating.<ref>[ | * [[profile consistency]], which is the observation that profiles tend to have roughly the same shape, regardless of the power and location of the applied heating.<ref>F. Ryter et al., [[doi:10.1088/0741-3335/43/12A/325|Plasma Phys. Control. Fusion '''43''', A323 (2001)]]</ref> | ||
* the [[Scaling law|Bohm scaling]] of confinement in L-mode (scaling of transport with system size) <ref>[ | * the [[Scaling law|Bohm scaling]] of confinement in L-mode (scaling of transport with system size) <ref>B.A. Carreras, [[doi:10.1109/27.650902|IEEE Trans. Plasma Science '''25''', 1281 (1997)]]</ref>, and | ||
* power degradation, as reflected in global transport [[Scaling law|scaling laws]]. The scaling of the plasma energy content with injected power is generally found to be sub-linear, i.e., considerably worse than expected from simple diffusion. | * power degradation, as reflected in global transport [[Scaling law|scaling laws]]. The scaling of the plasma energy content with injected power is generally found to be sub-linear, i.e., considerably worse than expected from simple diffusion. | ||
The basic explanation for these phenomena is self-regulation of the profiles by turbulence (see [[Anomalous transport]]). | The basic explanation for these phenomena is self-regulation of the profiles by turbulence (see [[Anomalous transport]]). | ||
<ref> | <ref>D.E. Newman et al., [[doi:10.1063/1.871681|Phys. Plasmas '''3''', 1858 (1996)]]</ref> | ||
The strong temperature and density gradients in fusion-grade plasmas provide free energy that may drive turbulence. The turbulence then enhances transport locally, leading to a local reduction of gradients and a consequential damping of the turbulence amplitude. This feedback could be responsible for keeping the gradients below a critical value. Considered locally, the former is a description of a simple marginal state. | The strong temperature and density gradients in fusion-grade plasmas provide free energy that may drive turbulence. The turbulence then enhances transport locally, leading to a local reduction of gradients and a consequential damping of the turbulence amplitude. This feedback could be responsible for keeping the gradients below a critical value. Considered locally, the former is a description of a simple marginal state. | ||
But the interaction of such feedback mechanisms at various radial locations would lead to ''avalanche'' behaviour and a true (scale-free) self-organised state. | But the interaction of such feedback mechanisms at various radial locations would lead to ''avalanche'' behaviour and a true (scale-free) self-organised state. | ||
Indeed, there is direct evidence for avalanching behaviour in numerical simulations | Indeed, there is direct evidence for avalanching behaviour in numerical simulations | ||
<ref> | <ref>L. García and B.A. Carreras, [[doi:10.1063/1.2041614|Phys. Plasmas '''12''', 092305 (2005)]]</ref>, | ||
but experimental evidence is scarce. | but experimental evidence is scarce. | ||
<ref> | <ref>P.A. Politzer, [[doi:10.1103/PhysRevLett.84.1192|Phys. Rev. Lett. '''84''', 1192 (2000)]]</ref> | ||
However, some indirect evidence exists. Typically, such evidence involves the detection of [[Long-range correlation|long-range correlations]] in fluctuations. | However, some indirect evidence exists. Typically, such evidence involves the detection of [[Long-range correlation|long-range correlations]] in fluctuations. | ||
<ref> | <ref>B.A. Carreras et al., [[doi:10.1063/1.873490|Phys. Plasmas '''6''', 1885 (1999)]]</ref> | ||
Evidence for critical gradients is much more abundant. | Evidence for critical gradients is much more abundant. | ||
<ref> | <ref>D.R. Baker et al., [[doi:10.1063/1.1395567|Phys. Plasmas '''8''', 4128 (2001)]]</ref> | ||
<ref>[ | <ref>F. Ryter et al., [[doi:10.1088/0741-3335/43/12A/325|Plasma Phys. Control. Fusion '''43''', A323 (2001)]]</ref> | ||
However, the existence of a critical gradient by itself does not prove the system is in a SOC state. | However, the existence of a critical gradient by itself does not prove the system is in a SOC state. | ||
==References== | ==References== | ||
<references /> | <references /> | ||